The increase in demand for maximum battery run-time and performance in portable devices translates directly into the increase in demand for high efficiency and low noise switch-mode voltage regulator circuits. Typically, the power supply system in a portable device includes a single battery source connected to a switch mode voltage regulator circuit and to a load. Since battery capacity is limited in portable electronic devices, a switch-mode voltage regulator circuit must have power efficiency to extend battery run-time. There are many attempts in prior arts to improve power efficiency in switch mode voltage regulator circuits to extend battery run-time.
Referring now to FIG. 1, a prior art switch mode voltage regulator circuit 100 is shown. In prior art switch mode voltage regulator circuit 100, power efficiency is improved by employing a sleep mode method. That is, switch mode regulator circuit 100 generates a control signal to turn one or both MOSFET switching transistors 141 and 142 off under operating conditions when the voltage at the output terminal (VOUT) is capable of being maintained substantially at the regulated voltage level by the charge in the output capacitor (COUT) 152. Even though the sleep mode method may achieve a relatively high power efficiency, high ripple voltages caused by the turning on and off the MOSFET switching transistors 141 and 142 often degrade the performance of switch mode voltage regulator circuit 100. Furthermore, turning on and off the MOSFET switching transistors 141 and 142 randomly, based on the load current (IL), causes uneven on-time pulses that produce spurious signals, thus worsening the noise performance of prior art switch mode voltage regulator circuit 100. In other words, such sleep mode method presents severe output voltage variations over a wide operation range. Especially, in today's portable devices, the microprocessors, as the loads for switch mode voltage regulator circuit 100, often transition from sleep mode to full operation in a matter of nanoseconds, thus presenting a steep load step and uneven pulse that increases the ripple voltage and undermines the noise performance at the output of switch mode voltage regulator circuit 100.
In addition, there are multiple ways to regulate power efficiently in a switch mode voltage regulator circuit. The first way is to provide a constant on-time to the high-side switch which decreases output ripple voltage and noise. The second way is to improve switching efficiency. Other ways to regulate power include synchronous rectification, zero-voltage switching (ZVS), mode-hopping, and variable frequency operation. Depending on the operating conditions, mode selection by a user-controlled pin should be allowed to achieve best efficiency performance. Prior art switching regulator circuit 100 only allows one way to regulate power simply by monitoring the load current and issuing a control signal to turn off both MOSFET switching transistors 141 and 142 to save energy. Whereas other methods of power saving techniques cannot be selected by the user.